Magnetic tape

ABSTRACT

A magnetic tape ( 1 ) characterized by having a backcoating layer ( 5 ) which comprises a binder and fine particles having been dispersed in the binder and being capable of irreversibly changing in color on oxidation reaction, and has a sufficient number of microvoids of sufficient size to supply sufficient oxygen to cause the oxidation reaction.

TECHNICAL FIELD

[0001] The present invention relates to magnetic tape having opticalservo tracks. More particularly, it relates to magnetic tape havingoptical servo tracks on the side opposite to the magnetic recordingside.

BACKGROUND ART

[0002] The recent expanding scale of the computer network and theimportance of security for data management have been increasing thedemand for magnetic tape having an increased recording capacity for useas a medium for data backup. Approaches to high recording capacity aredivided into improvement on recording density and extension of the tapelength.

[0003] Since the tape length that can be put in a tape cartridge aswound is the upper limit of the recording capacity, extension of thetape length for increasing the recording capacity cannot be achieved butby reducing the tape thickness. Therefore, an increase in recordingcapacity attained by this approach is of necessity limited. With respectto the method of increasing a recording density, it is known thatmagnetic tape has a lower recording density than a hard disc drive.Serpentine type magnetic tape particularly has a low recording density,which is due to the low track density. On the other hand helical scantype magnetic tape is known to have a higher track density than theserpentine type magnetic tape. This is because the magnetic tape ofhelical scan type uses a servo tracking system called automatic trackfinding (ATF).

[0004] A servo tracking system has also been adopted to serpentine typemagnetic tape to improve the track density. Methods that have beenproposed as such a servo tracking system include an embedded servosystem, in which servo signals are written on the same track as the datatrack on the magnetic recording surface, and a system in which a trackexclusive to servo signals is provided on the magnetic recordingsurface. Japanese Patent Publication No. 82626/95 proposes a trackingsystem particularly useful where the pitch of data tracks is as small asseveral tens of microns, in which a dedicated track for servoinformation is provided on the magnetic recording surface, and aplurality of servo signal reproduction heads are used to read the servosignals for tracking. According to this technique, however, the numberof servo signal reproduction heads must be increased as the number oftracks increases. In order to avoid this, the servo track should beincreased. Like this, conventional servo tracking systems use the sameside of magnetic tape as used for data recording, which results inreduction of the data recording area. This problem is conspicuous in theservo tracking system of Japanese Patent Publn. No. 82626/95 when atrack density is as high as about 30 tpmm (tracks per mm) or even more.

DISCLOSURE OF THE INVENTION

[0005] Accordingly, an object of the present invention is to providemagnetic tape which is capable of servo tracking without reducing thedata area.

[0006] Another object of the present invention is to provide magnetictape having an increased track density.

[0007] Still another object of the present invention is to providemagnetic tape having a high recording capacity.

[0008] As a result of extensive investigation, the inventors of thepresent invention have found that magnetic tape accomplishing the aboveobjects can be obtained by incorporating specific fine particles intothe backcoating layer of the magnetic tape and forming specific voids inthe backcoating layer to make the backcoating layer capable of formingservo tracks.

[0009] Completed based on the above finding, the present inventionaccomplished the above objects by providing magnetic tape comprising asubstrate, a magnetic layer provided on one side of the substrate and abackcoating layer provided on the other side of the substrate, whereinthe backcoating layer comprises a binder and fine particles having beendispersed in the binder and being capable of irreversibly changing incolor on oxidation reaction, and has a sufficient number of microvoidsof sufficient size to supply sufficient oxygen to cause the oxidationreaction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Various other objects, features and attendant advantages of thepresent invention will be better understood from the followingdescription and the accompanying drawings, in which like referencecharacters designate like parts and wherein:

[0011]FIG. 1 is a schematic view showing the structure of one embodimentof the magnetic tape according to the present invention.

[0012]FIG. 2 schematically illustrates a method for forming a colorchange pattern by irradiating a backcoating layer with light beams.

[0013]FIG. 3 is an enlarged partial view of the backcoating layerirradiated with light beams.

[0014] FIGS. 4(a), 4(b), 4(c), and 4(d) schematically illustrate amethod for carrying out servo tracking by a push-pull method.

[0015]FIG. 5 schematically shows another color change pattern(corresponding to FIG. 3).

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] The magnetic tape of the present invention will be described withreference to the preferred embodiments thereof by referring to theaccompanying drawings, in which FIG. 1 is a schematic view showing thestructure of an embodiment of the magnetic tape according to the presentinvention, FIG. 2 schematically illustrates a method for forming a colorchange pattern by irradiating a backcoating layer with light beams, andFIG. 3 is an enlarged partial plane view of the backcoating layerirradiated with light beams.

[0017] Magnetic tape 1 of the embodiment shown in FIG. 1 comprises asubstrate 2 having provided thereon an intermediate layer 3 and amagnetic layer 4 as a top layer adjoining the intermediate layer 3. Thesubstrate 2 has on the other side a backcoating layer 5.

[0018] The magnetic tape 1 shown in FIG. 1 is used for a serpentinerecording system. The magnetic layer 4 has a plurality of data tracks inparallel with the running direction of the magnetic tape 1. On use, ahead unit having a predetermined number of magnetic heads is movedacross the magnetic tape 1, switching among data tracks, to record orreproduce data on the data track corresponding to each magnetic head.Servo tracking is carried out so that each magnetic head may bepositioned on a right data track on switching among the tracks or duringrecording or reproduction.

[0019] The backcoating layer 5 is formed of a binder having dispersedtherein fine particles that change its color irreversibly on beingoxidized. Oxidation reaction of the fine particles can be induced byaffording energy necessary for the reaction. While the method ofaffording energy is not particularly limited, a method in which energycan be given only to a specific small area is preferably used. Such amethod includes irradiation with a light beam, such as a laser beam.

[0020] The manner of irradiating the backcoating layer 5 with a lightbeam to oxidize the fine particles is explained by referring to FIG. 2.

[0021] As shown in FIG. 2, a plurality of laser beams 41 are emitted inparallel from the respective laser light sources 40 aligned atprescribed intervals across the width direction of the magnetic tape 1and illuminate the backcoating layer 5 of the magnetic tape 1 running indirection A at a predetermined speed. The fine particles present in theparts irradiated with the laser beams 41 undergo oxidation reaction withoxygen present in air and change in color. The irradiation conditionswith the laser beams 41 are controlled so that the color change mayoccur over the whole thickness of the irradiated part of the backcoatinglayer 5. The color change provides a prescribed color change pattern 10in the backcoating layer 5. The color change pattern formed in thisparticular embodiment is comprised of a plurality of continuous lines ofprescribed width along the longitudinal direction of the magnetic tape 1as illustrated in FIG. 2. The width w of each line of the color changepattern 10 and the degree of color change in the thickness direction ofthe backcoating layer 5 can be adjusted by controlling the beam diameterand output of the laser beams 41. In this embodiment, the beam diameteris preferably 0.25 to 30 μm, particularly 1 to 25 μm, and the output ispreferably 0.02 to 2 W, particularly 0.02 to 0.5 W. The color changepattern 10 in FIG. 2 is magnified.

[0022]FIG. 3 is referred to for going into details of the color changepattern thus formed. The color change pattern 10 is comprised ofstraight lines having a prescribed width w, arrayed in parallel to eachother in the longitudinal direction of the magnetic tape 1 and spacedequally in the width direction of the magnetic tape 1. The color changepattern 10 is formed over the whole length of the magnetic tape 1. Thecolor change pattern 10 is such that makes an optical contrast so thatservo tracking for the data tracks on the magnetic layer 4 may becarried out based on the optical information provided from the colorchange pattern 10. As stated above, while the data tracks on themagnetic layer 4 are also formed in parallel to the longitudinaldirection of the magnetic tape 1 similarly to the color change pattern10, the relative positional relationship between the data tracks and thecolor change pattern 10 is not particularly limited.

[0023] The optical contrast made by the above-described color changepattern 10 includes a contrast of intensity of transmitted light whenlight of prescribed wavelength is incident on the color change pattern10 and a contrast of intensity of reflected light when light ofprescribed wavelength is incident on the color change pattern 10.

[0024] Where the contrast of transmitted light intensity is used forservo tracking, the intensity of transmitted light is detected toconduct servo tracking by an optical servo system, such as a push-pullmethod or a three-beam method. In using the contrast of reflected lightintensity, the intensity of reflected light is detected to carry outservo tracking by the above-described servo system in the similarmanner. The optical servo systems, such as a push-pull method and athree-beam method, are techniques commonly employed for achieving servotracking in various optical discs.

[0025] Servo tracking based on the detected intensity of transmittedlight by, for example, a push-pull method is described by referring toFIG. 4. As shown in FIG. 4(a), light is emitted from a light source 30,such as a semiconductor laser, which is placed to face the backcoatinglayer 5 of the magnetic tape running in the direction perpendicular tothe surface of the paper, condensed through a lens 31 to a prescribedbeam diameter, and enters the color change pattern 10 formed on thebackcoating layer 5. The beam diameter is slightly smaller than the linewidth of the color change pattern 10. The intensity of the light havingbeen transmitted through the color change pattern 10, the substrate 2(not shown), the intermediate layer 3 (not shown), and the magneticlayer 4 (not shown), i.e., transmitted light is detected by a lightdetector 33. The transmitted light intensity is converted to electricalsignals and sent to a servo tracking processor 34. The symmetry of thetransmitted light beam intensity is processed in the servo trackingprocessor 34. If the beam intensity displays bilateral symmetry aboutthe center line of the beam, it means that the beam 35 is incident onthe center line of the color change pattern 10 as shown in FIG. 4(b).This state is an “on-track” state, that is, the magnetic head isproperly positioned on an aimed data track of the magnetic layer. If thebeam intensity lacks bilateral symmetry about the center line of thebeam, it indicates that the beam 35 is deviating from the center line toeither left or right as shown in FIG. 4(c) or (d). This state is an“off-track” state, that is, the magnetic head is not properly positionedon the data track of the magnetic layer. Then the servo trackingprocessor 34 gives a drive 35 of the magnetic head 36 instructions tomove the magnetic head 36 to a proper position as shown in FIG. 4(a). Asa result, the magnetic head 36 is properly positioned by the drive 35 torestore the “on-track” state.

[0026] As shown in FIG. 3, the line width w of the color change pattern10 is preferably 0.25 to 50 μm. If the line width w is smaller than 0.25μm, optical detection of the color change pattern may be disturbedbecause it is difficult with the state-of-the-art technique to condensethe beam sufficiently. If the line width w exceeds 50 μm, the density ofthe color change pattern 10 unfavorably decreases where the pattern iscomprised of a large number of lines as illustrated in FIG. 3.Therefore, the above-described range is preferred. A still preferredline width w of the color change pattern 10 is 0.25 to 30 μm,particularly 0.8 to 25 μm. In the present invention, it is preferred touse transmitted light for servo tracking. In that case, it is preferredfor the whole magnetic tape before color change (the magnetic layer, theintermediate layer, the substrate, and the backcoating layer as a whole)has a light transmission of 15 to 40% for servo tracking.

[0027] While depending on the number of the lines forming the colorchange pattern 10, it is preferred that the pitch p of the color changepattern 10, i.e., the pitch of the adjacent color change lines be notless than the width of the data track formed on the magnetic layer 4 andbe an integral multiple of the width of the data track.

[0028] The color change pattern 10 may be arranged over the whole widthof the magnetic tape 1 at prescribed intervals as shown in FIG. 3, or inpart of the width of the magnetic tape 10. For example, a plurality oflines spaced at prescribed intervals may be arranged in the centralportion or either one of side portions of the tape in the widthdirection. Further, a plurality of lines spaced at prescribed intervalsmay be arranged in two or more portions of the magnetic tape 10 in thewidth direction. For example, two groups of lines (each group consistsof at least one line, and the groups may consist of the same ordifferent number of lines) can be arranged on each side portion of thetape; two groups of lines (each group consists of at least one line, andthe groups may consist of the same or different number of lines) can bearranged on the central portion and one of the side portions of thetape; or three groups of lines (each group consists of at least oneline, and the groups may consist of the same or different number oflines) can be arranged on the central portion and each side portion ofthe tape. In any case, the total number of the lines making up the colorchange pattern 10 is preferably a measure of the number of the datatracks of the magnetic layer 4.

[0029] The backcoating layer 5 has microvoids the number and the size ofwhich are sufficient for supplying sufficient amount of oxygen to induceoxidation reaction of the above-described fine particles. Oxygen issupplied through the microvoids to the whole thickness of thebackcoating layer 5 thereby making the fine particles undergo sufficientoxidation reaction. As a result, there is formed a color change pattern10 providing sufficient optical contrasts. The microvoids may be eitheropen pores exposed on the surface of the backcoating layer 5 or closedpores which exist inside the backcoating layer 5 and are not exposed onthe surface. However, if there are too many closed pores, the amounts ofvarious particles such as the above-described fine particles and abinder per unit volume are reduced relatively, which tends to make thecontrasts of the color change pattern insufficient or make the filmstrength of the backcoating layer S insufficient. Accordingly, it ispreferred that the microvoids are open pores or most of the microvoidsare open pores. As long as the contrasts of the color change pattern andthe film strength of the backcoating layer 5 retain sufficient levels,it is not at all problematical that the microvoids exist in a closedstate.

[0030] Microvoids can be formed in the backcoating layer 5 bycontrolling the weight ratio of the total amount of various particleshereinafter described (i.e., the total amount of all inorganic particlescontained in the backcoating layer 5) to the total resinous contentincluding a binder, a hardener, etc. (hereinafter referred to as P/Bratio). A preferred P/B ratio is 100/10 (=10) to 100/30 (=3.33),particularly 100/14 (=7.14) to 100/25 (=4). With the P/B ratio of thebackcoating layer 5 being within this range, it is possible to formmicrovoids preferably having a diameter of 1 to 20 nm, particularly 2 to15 nm, and a void volume (volumetric ratio of the microvoids in thevolume of the backcoating layer 5) of 5 to 40% by volume, particularly10 to 35% by volume.

[0031] The diameter and volume of the microvoids are measured by anitrogen adsorption method according to the following procedure.

[0032] A high-precision automatic gas adsorption apparatus “BELSORP 36”manufactured by Nippon Bell K.K. is used as measuring equipment.

[0033] A piece measuring about 100 cm² is taken out of a magnetic tapehaving only the backcoating layer side left on the substrate (i.e., amagnetic tape from which the magnetic layer 4 and the intermediate layer3 have been removed), which was used as a sample of measurement. Thesample is sealed into a sample tube. Nitrogen having a purity of99.9999% and helium having a purity of 99.99999% are used as anadsorbing gas and a carrier gas, respectively.

[0034] The sample is allowed to stand at room temperature for 1 hour(reached degree of vacuum: 0.2 to 0.4 Pa) prior to the measurement, andthen measurement is made at an adsorption temperature of 77 K. Themeasurement mode is an isothermal adsorption-desorption mode. Themeasuring range is from 0.00 to 0.99 in terms of relative pressure(P/P₀), and the equilibrating time is 300 seconds for every relativepressure.

[0035] The distribution of the measured void diameters is calculated bya DH (Dollimore & Heal) method and smoothed. Prior to the measurement ofthe sample, measurement is made on graphite carbon available from NPL(National Physical Laboratory), an international standard sample (proofvalue: 11.1 m²/g; σ=0.8 m²/g), to confirm that the precision andaccuracy of measurement are within 2% and within 5%, respectively. Novoids are present in the substrate.

[0036] The terminology “(void) diameter” as used herein means the voiddiameter at which the distribution curve obtained from the measurementof void diameter reaches the maximum peak (the highest frequency in thedistribution curve).

[0037] The “void volume” is a value obtained by dividing the totalvolume of the microvoids calculated by the above-described DH method bythe volume of the backcoating layer (the product of the thickness andthe area) and multiplying the quotient by 100.

[0038] The above-described fine particles are now described in detail.

[0039] Any fine particles that undergo irreversible color change onbeing oxidized can be used with no particular restriction. It isparticularly preferred to use metal oxides for their readiness todiscoloration and the color contrast produced by the discoloration. Themetal oxides include, for example, FeO_(x) (1.34<x<1.5), TiO, SnO, MnO,and Cr₂O₃. It is particularly preferred to use FeO_(x) for itssatisfactory discoloration properties.

[0040] FeO_(x) is iron oxide of magnetite type comprising divalent Feand trivalent Fe. It is preferred for the FeO_(x) to have a divalent Fecontent of 5 to 24% by weight, especially 10 to 20% by weight, based onthe total FeO_(x).

[0041] The fine particles preferably have a primary particle size of 1to 200 nm, particularly 5 to 80 nm, from the viewpoint of the surfacesmoothness of the backcoating layer. For the consideration of theabove-mentioned P/B ratio, it is preferred that the fine particles bepresent in an amount of 300 to 1200 parts by weight, particularly 350 to1000 parts by wight, per 100 parts by weight of the binder. Morespecifically, where the amount of the fine particles is less than 300parts by weight, the sensitivity to color change tends to beinsufficient for obtaining optically sufficient contrasts. If it exceeds1200 parts by weight, the coating film of the backcoating layer tends tohave reduced strength. Therefore, the above-described range ispreferred.

[0042] Any binders can be used with no restriction as long as applicableto magnetic tape. For example, thermoplastic resins, thermosettingresins, reactive resins, and mixtures thereof can be used. Specificexamples are vinyl chloride copolymers or modified vinyl chloridecopolymers, copolymers comprising acrylic acid, methacrylic acid oresters thereof, acrylonitrile copolymers (rubbery resins), polyesterresins, polyurethane resins, epoxy resins, cellulosic resins, andpolyamide resins. These binders preferably have a number averagemolecular weight of 2,000 to 200,000. The binder resin can have apolarizing functional group (so-called polar group), such as a hydroxylgroup, a carboxyl group or a salt thereof, a sulfoxyl group or a saltthereof, a phosphate group or a salt thereof, a nitro group, a nitricester group, an acetyl group, a sulfuric ester group or a salt thereof,an epoxy group, a nitrile group, a carbonyl group, an amino group, analkylamino group, an alkylammonium salt group, and a betaine structure,such as sulfobetaine or carbobetaine, to have improved dispersingproperties for various particles which are incorporated into thebackcoating layer 5.

[0043] While the backcoating layer 5 in the magnetic tape 1 serves toform color change pattern used for servo tracking as mentioned above, itis a matter of course that it should have the functions essential to abackcoating layer. Such functions include (1) providing magnetic tapewith satisfactory running properties, (2) providing magnetic tape withantistatic properties, and (3) detecting the beginning (BOT) and the end(EOT) of the tape.

[0044] To perform the function (1), it is preferred for the backcoatinglayer 5 to have a moderate surface roughness. On the other hand, it ispreferred for the backcoating layer 5 to be as smooth as possible toprevent the surface profile of the backcoating layer 5 from beingtransferred to the magnetic layer while the tape is wound. Taking thebalance between these requirements into consideration, the backcoatinglayer 5 preferably has an arithmetic mean roughness Ra of 7 to 50 nm,particularly 8 to 30 nm, and a 10 point mean roughness Rz of 40 to 250nm, particularly 50 to 200 nm.

[0045] The arithmetic mean roughness Ra, defined by the followingequation (i), was measured with a stylus-type profilometer under thefollowing conditions in accordance with JIS-B0601-1994. Stylus:diameter: 1.5 to 2.5 μm; curvature: 60° Contact pressure:  50 to 300 μNCut-off length:  80 μm Sampling length:  80 μm Assessment length: 400 μm

[0046] $\begin{matrix}{{Ra} = {\frac{1}{l}{\int_{0}^{l}{{{y(x)}}\quad {x}}}}} & (i)\end{matrix}$

[0047] wherein Y represents profile data; and I represents an assessmentlength.

[0048] In carrying out the measurement, a sample piece is stuck to aslide glass for microscopes which satisfies the requirements specifiedin JIS-R-3502 (while, in the present invention, a slide glass producedby Matsunami Glass K.K. was used, usable slide glass is not limitedthereto) with water or ethanol. Existence of excessive water or ethanolwill ruin the reproducibility of measurements. Therefore, the resultsobtained after the water or ethanol evaporates to some extent and whilean interference fringe can be seen from the back of the slide glass aretaken as Ra.

[0049] The 10 point mean roughness Rz, being defined by the followingequation (ii), was obtained under the same conditions as for themeasurement of Ra in accordance with JIS-B0601-1994. The sample piecewas the same as used for Ra; the sampling length l was 80 μm, and theassessment length l₀ was 400 μm. $\begin{matrix}{{Rz} = \frac{{{Y_{p1} + Y_{p2} + Y_{p3} + Y_{p4} + Y_{p5}}} + {{Y_{v1} + Y_{v2} + Y_{v3} + Y_{v4} + Y_{v5}}}}{5}} & ({ii})\end{matrix}$

[0050] wherein Y_(p1), Y_(p2), Y_(p3), Y_(p4) and Y_(p5) are heights ofthe five highest peaks within the sampled section corresponding to thesampling length l; and Y_(v1), Y_(v2), Y_(v3), Y_(v4) and Y_(v5) areheight of the five lowest valleys within the sampled sectioncorresponding to the sampling length l.

[0051] In order for the backcoating layer 5 to have the arithmetic meanroughness Ra and the 10 point mean roughness Rz within theabove-specified respective preferred ranges, it is preferable for thebackcoating layer 5 to contain carbon black having a primary particlesize of 15 to 80 nm, a BET specific surface area of 10 to 80 m²/g, and aDBP oil absorption of 100 to 300 cm³/100 g. It is still preferred forthe carbon black to have a primary particle size of 25 to 80 nm, a BETspecific surface area of 15 to 70, and a DBP oil absorption of 120 to250 cm³/100 g.

[0052] The carbon black is preferably incorporated in an amount of 0.1to 5 parts by weight, particularly 0.1 to 3 parts by weight, per 100parts by weight of the binder, which is effective for obtaining theabove-described preferred P/B ratio and the above-described preferredarithmetic mean roughness Ra and 10 point mean roughness Rz of thebackcoating layer 5.

[0053] Carbon black is known to have high light shielding properties. Ifcarbon black is added to the backcoating layer 5 in a large quantity,the layer will have high light shielding properties and may fail totransmit sufficient light, which is unfavorable where transmitted lightis made use of for servo tracking. Such being the case, it is apreferred manipulation to incorporate, into the backcoating layer 5,silicone resin particles whose primary particle size is smaller than thethickness of the backcoating layer 5 in place of or in combination with,the carbon black, thereby to achieve the function (1). The siliconeresin particles preferably have a primary particle size of 10 to 500 nm,particularly 10 to 300 nm. Silicone resin particles which can be usedsuitably include, for example, alkyl-modified silicone resins (resinparticles having siloxane bonds extending in three dimensions to form anetwork structure in the inside thereof, and having the terminals of thenetwork structure, i.e., the surface of the particles, modified with analkyl). The silicone resin particles are preferably incorporated in anamount of 0.05 to 10 parts by weight, particularly 0.1 to 5 parts byweight, per 100 parts by weight of the binder, irrespective of whetherthe silicone resin particles are used alone or in combination with thecarbon black, which range is preferred for obtaining the above-describedpreferred P/B ratio and the above-described preferred arithmetic meanroughness Ra and 10 point mean roughness Rz of the backcoating layer 5.

[0054] To perform the function (2), it is preferred for the backcoatinglayer 5 to contain an electrically conductive substance. Although theabove-mentioned carbon black is a typical example of such a substance,incorporation of a large amount of carbon black into the backcoatinglayer 5 results in increased light shielding properties, and sufficientlight cannot be transmitted as stated above in cases where transmittedlight is used for servo tracking. This being the case, it is a preferredembodiment to use electrically conductive inorganic particles in placeof, or in combination with carbon black thereby to obtain the function(2). The electrically conductive inorganic particles include thosedescribed in Japanese Patent Laid-Open No. 236541/94, col. 3, ll. 42-45,such as tin oxide, titanium dioxide, zinc oxide, indium oxide, zincsulfide, barium sulfate, silicon oxide, and magnesium carbonate. Theseelectrically conductive inorganic particles are generally white,assuring high light transmitting properties, which affords anotheradvantage where transmitted light is utilized for servo tracking.Especially preferred electrically conductive inorganic particles are tinoxide, antimony-doped tin oxide (ATO), indium-doped tin oxide (ITO), andindium oxide. These electrically conductive inorganic particlespreferably have a primary particle size of 1 to 100 nm, particularly 2to 100 nm, especially 5 to 50 nm. These electrically conductiveinorganic particles are preferably added in an amount of 5 to 100 partsby weight, particularly 10 to 80 parts by weight, per 100 parts byweight of the above-described fine particles irrespective of whether theelectrically conductive inorganic particles are used alone or incombination with the carbon black, which range is preferred forobtaining the above-described preferred P/B ratio and for sufficientlyperforming the function (2).

[0055] In the magnetic tape according to the present invention, thefunction (3) can be performed substitutionally by the color changepattern 10. EOT or BOT has conventionally been detected by a lighttransmission method so that it has been essential for the backcoatinglayer 5 to contain carbon black. Incorporation of carbon black fordetection of EOT or BOT is unnecessary in the present invention. Thisoffers an extreme advantage where transmitted light is used for servotracking.

[0056] The backcoating layer 5 can contain a lubricant, a hardener, andthe like in addition to the aforementioned components.

[0057] Fatty acids and fatty acid esters are commonly used as alubricant.

[0058] Examples of the fatty acids are caproic acid, caprylic acid,capric acid, lauric acid, myristic acid, palmitic acid, stearic acid,isostearic acid, linolenic acid, oleic acid, elaidic acid, behenic acid,malonic acid, succinic acid, maleic acid, glutaric acid, adipic acid,pimelic acid, azelaic acid, sebacic acid, 1,12-dodecanedicarboxylicacid, and octanedicarboxylic acid.

[0059] Examples of the fatty acid esters are alkyl esters of theabove-enumerated fatty acids, with those having 16 to 46 carbon atoms intotal being preferred.

[0060] Inorganic acid esters, such as phosphoric esters, are also usefulas a lubricant.

[0061] The lubricant is added in an amount of 0.05 to 15 parts byweight, preferably 0.2 to 10 parts by weight, per 100 parts by weight ofthe binder.

[0062] The hardeners to be used generally include isocyanate hardeners,exemplified by “Coronate L” (a trade name, produced by NipponPolyurethane Industry Co., Ltd.) and amine hardeners. The hardener isadded in an amount of 5 to 30 parts by weight, preferably 5 to 20 partsby weight, per 100 parts by weight of the binder.

[0063] The backcoating layer 5 is formed by coating the substrate 2 witha backcoating composition having the above-mentioned componentsdispersed in a solvent. The solvents include ketone solvents, estersolvents, ether solvents, aromatic hydrocarbon solvents, and chlorinatedhydrocarbon solvents. The solvent is preferably used in such an amountthat the backcoating composition may have a solids content of 10 to 50%by weight, particularly 20 to 40% by weight.

[0064] The thickness of the backcoating layer 5, formed by applying thebackcoating composition, is preferably 0.1 to 1.5 μm, still preferably0.2 to 0.8 μm, taking into consideration the light transmission of thecolor change pattern 10 and the thickness balance with the magneticlayer 4 and the intermediate layer 3, and the like.

[0065] The backcoating layer 5 shown in FIG. 3 has a color changepattern 10 of a plurality of lines along the longitudinal direction ofthe magnetic tape 1. In place of such a pattern, a color change patternof a single continuous straight line may be formed on the backcoatinglayer S along the longitudinal direction of the magnetic tape 1. Thepattern to be formed on the backcoating layer 5 may be a single or aplurality of continuous sine curves along the longitudinal direction ofthe magnetic tape 1. Further, the pattern 10 can be comprised ofdiscontinuous pieces of lines along the longitudinal direction of themagnetic tape 1 as shown in FIG. 5.

[0066] The color change pattern 10 shown in FIG. 5 is described below.The color change pattern 10 is made up of pieces 10 a angled at Θ° withthe longitudinal direction of the magnetic tape 1 and pieces 10 b angledat −Θ°, which alternate with each other along the centerline c of themagnetic tape 1. The angle Θ has an influence on the accuracy ofpositioning by servo tracking. A preferred angle Θ for securingsufficient accuracy of positioning is 5 to 85°, particularly 10 to 30°.The lengths of the pieces 10 a and 10 b may be the same or different butare preferably the same. A preferred length of the pieces 10 a and 10 bis 5 to 140 mm, particularly 5 to 80 mm. The spacing between the piece10 a and the piece 10 b, in terms of the interval g along thelongitudinal direction of the magnetic tape 1, is preferably as narrowas possible. Servo tracking based on the color change pattern 10 shownin FIG. 5 can be carried out in the same manner as in the case of usingthe color change pattern 10 shown in FIG. 3.

[0067] General particulars concerning the magnetic tape according to thepresent invention are described hereunder.

[0068] The magnetic layer 4 of the magnetic tape 1 shown in FIG. 1 isformed by applying a magnetic coating composition comprisingferromagnetic powder and a binder. Namely, the magnetic tape 1 isparticulate magnetic tape.

[0069] The ferromagnetic powder which can be used include acicular orspindle-shaped ferromagnetic powder and tabular ferromagnetic powder.Acicular or spindle-shaped ferromagnetic powder includes ferromagneticmetal powder mainly comprising iron and ferromagnetic iron oxide powder,and tabular ferromagnetic powder includes ferromagnetic hexagonalferrite powder.

[0070] More specifically, the ferromagnetic metal powder includes powderhaving a metal content of 50% by weight or more, 50% by weight or moreof the metal content being iron. Specific examples of such ferromagneticmetal powders include Fe—Co, Fe—Ni, Fe—Al, Fe—Ni—Al, Fe—Co—NiFe—Ni—Al—Zn, and Fe—Al—Si. The ferromagnetic iron oxide powder includesγ-Fe₂O₃, Co-doped γ-Fe₂O₃, and Co-doped FeO_(x) ({fraction(4/3)}≦x<1.5). The acicular or spindle-shaped ferromagnetic powderpreferably has a major axis length of 0.03 to 0.2 μm, particularly 0.05to 0.16 μm, with an acicular ratio (major axis length/minor axis length)of 3 to 15, particularly 3 to 10. The acicular or spindle-shapedferromagnetic powder preferably has a coercive force (Hc) of 125 to 200kA/m, particularly 135 to 190 kA/m, and a saturation magnetization (σs)of 119 to 167 Am²/kg, particularly 127 to 152 Am^(2/)kg. Further, theBET specific surface area of the acicular ferromagnetic powder ispreferably 30 to 70 m²/g, particularly 40 to 70 m²/g.

[0071] The ferromagnetic hexagonal ferrite powder includes fine tabularparticles of barium ferrite or strontium ferrite, part of the Fe atomsof which may be displaced with Ti, Co, Ni, Zn, V or the lie atoms. Theferromagnetic hexagonal ferrite powder preferably has a tabular diameterof 0.1 μm or smaller, particularly 10 to 90 nm, especially 10 to 40 nm,and an aspect ratio (diameter/thickness) of 2 to 7, particularly 2 to 5.It preferably has a coercive force (Hc) of 135 to 260 kA/m and asaturation magnetization (σs) of 27 to 72 Am²/kg, particularly 43 to 72Am²/kg. Further, the ferromagnetic hexagonal ferrite powder preferablyhas a BET specific surface area of 30 to 70 m²/g.

[0072] If necessary, the ferromagnetic powder can contain rare earthelements or transition metal elements. The ferromagnetic powder can besubjected to a surface treatment to improve dispersibility and the like.The surface treatment can usually be performed by a method similar tothe method for coating the surface of the ferromagnetic powder with aninorganic oxide. Inorganic oxides which can be used in this surfacetreatment include Al₂O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂O₃, and ZnO. Theseinorganic oxides can be used either individually or as a mixture of twoor more thereof. The surface treatment can also be carried out by anorganic treatment, such as a silane coupling treatment, a titaniumcoupling treatment or an aluminum coupling treatment.

[0073] The binder to be used can be of those illustrated for theformation of the backcoating layer 5. While the details of the binderare not described here, the explanations given in relation to thebackcoating layer 5 apply appropriately. The binder is preferably usedin an amount of 10 to 40 parts by weight, particularly 15 to 25 parts byweight, per 100 parts by weight of the ferromagnetic powder.

[0074] The magnetic layer 4 can further contain abrasive grains, carbonblack, lubricants, hardeners, etc. in addition to the aforementionedcomponents.

[0075] The abrasive grains that are preferably used include particles ofsubstances having a Mohs hardness of 7 or higher, such as alumina,silica, ZrO₂, and Cr₂O₃. From the standpoint of reduction in frictionalcoefficient during running and improvement in running durability, theabrasive grains preferably have a primary particle size of 0.03 to 0.6μm, particularly 0.05 to 0.3 μm. The abrasive grains are preferablyadded in an amount of 2 to 20 parts by weight, particularly 3 to 15parts by weight, per 100 parts by weight of the ferromagnetic powder.

[0076] The carbon black, lubricant, and hardener to be used can be ofthose useful for the formation of the backcoating layer 5. Therefore,the details of these components are not described here. The explanationsgiven in relation to the backcoating layer 5 apply appropriately. Thecarbon black is preferably used in an amount of 0.1 to 10 parts byweight, particularly 0.1 to 5 parts by weight, per 100 parts by weightof the ferromagnetic powder. The lubricant is preferably used in anamount of 0.5 to 10 parts by weight, particularly 0.5 to 5 parts byweight, per 100 parts by weight of the ferromagnetic powder. Thehardener is preferably used in an amount of 2 to 30 parts by weight,particularly 5 to 20 parts by weight, per 100 parts by weight of thebinder.

[0077] If desired, the magnetic layer 4 can contain various additivescustomarily used in magnetic tape, such as dispersants, rust inhibitors,and antifungals, in addition to the above-described components.

[0078] The magnetic layer 4 is formed by applying a magnetic coatingcomposition having the aforementioned components dispersed in a solventon an intermediate layer 3. The solvent can be of those illustrated foruse in the backcoating composition. The solvent is preferably used in anamount of 80 to 500 parts by weight, particularly 100 to 350 parts byweight, per 100 parts by weight of the ferromagnetic powder present inthe magnetic coating composition.

[0079] The magnetic coating composition is prepared by, for example,preliminarily mixing the ferromagnetic powder and the binder togetherwith a portion of the solvent in a Naughter mixer, etc., kneading thepremixture in a continuous pressure kneader, a twin-screw kneadingmachine, etc., diluting the mixture with another portion of the solvent,followed by dispersing in a sand mill, etc., adding to the dispersionadditives, such as a lubricant, filtering the dispersion, and addingthereto the remainder of the solvent and a hardener.

[0080] The magnetic layer 4 formed of the above-described magneticcoating composition preferably has a coercive force of 119 to 280 kA/m,particularly 120 to 250 kA/m, especially 125 to 222 kA/m, to securesufficient recording and reproducing characteristics. Further, themagnetic layer 4 preferably has a saturation flux density of 0.1 to 0.5T, particularly 0.15 to 0.45 T.

[0081] For obtaining an improved S/N ratio and for preventingself-demagnetization, the thickness of the magnetic layer 3 ispreferably 0.05 to 3 μm, still preferably 0.1 to 0.8 μm.

[0082] The intermediate layer 3 is explained below.

[0083] The intermediate layer 3 may be either a layer having magnetismor a nonmagnetic layer. Where the intermediate layer 3 is a layer havingmagnetism, it is a magnetic layer containing magnetic powder, which isformed by using a magnetic coating composition mainly comprisingmagnetic powder, nonmagnetic powder, a binder, and a solvent. Where, onthe other hand, the intermediate layer 3 is a nonmagnetic layer, theintermediate layer 5 is formed by using a nonmagnetic coatingcomposition mainly comprising nonmagnetic powder, a binder, and asolvent (these coating compositions will be inclusively referred to asan intermediate layer coating composition).

[0084] Ferromagnetic powder is preferably used as the magnetic powder.Either of hard magnetic powder and soft magnetic powder can be usedpreferably.

[0085] The hard magnetic powder includes the ferromagnetic hexagonalferrite powder, ferromagnetic metal powder and ferromagnetic iron oxidepowder which can be used in the magnetic layer 4. The details of theseferromagnetic powders, while not described here, are the same as theferromagnetic powders used in the magnetic layer 4, and the explanationsgiven thereto apply appropriately.

[0086] While the soft magnetic powder to be used is not particularlylimited, magnetic powder generally used in so-called low-currentdevices, such as a magnetic head and an electron circuit, are preferred.For example, the soft magnetic materials described in ChikazumiToshinobu, “Kyojiseitai no Buturi (2nd Vol.) Jikitokusei to Ohyo”, pp.368-376, Shokabo (1984) can be used. Specifically, soft magnetic oxidepowder and soft magnetic metal powder can be used.

[0087] Spinel type ferrite powder is preferably used as the softmagnetic oxide powder. The spinel type ferrite powder includes MnFe₂O₄,Fe₃O₄, CoFe₂O₄, NiFe₂O₄, MgFe₂O₄, Li_(0.5)Fe_(2.5)O₄, Mn—Zn typeferrite, Ni—Zn type ferrite, Ni—Cu type ferrite, Cu—Zn type ferrite,Mg—Zn type ferrite, Li—Zn type ferrite, Zn type ferrite, and Mn typeferrite. These soft magnetic oxide powders may be used eitherindividually or as a combination of two or more thereof.

[0088] The soft magnetic metal powder includes Fe—Si alloys, Fe—Alalloys (e.g., Alperm, Alfenol and Alfer), Permalloy (e.g., Ni—Fe binaryalloys or mulinary alloys composed of the Ni—Fe binary system and Mo,Cu, Cr, etc.), Sendust (Fe—Si(9.6 wt %)-Al(5.4 wt %)), and Fe—Co alloys.These soft magnetic metal powders may be used either individually or asa combination of two or more thereof.

[0089] The soft magnetic oxide powder usually has a coercive force of 8to 12000 A/m and a saturation magnetization of 30 to 90 Am²/kg. The softmagnetic metal powder usually has a coercive force of 1.6 to 8000 A/mand a saturation magnetization of 5 to 500 Am2/kg.

[0090] While not limiting, the shape of the soft magnetic powdersinclude a spherical shape, a tabular shape, and an acicular shape. Thesize of the particles is preferably 5 to 800 nm.

[0091] If desired, the above-described magnetic powder can contain rareearth elements or transition metal elements similarly to theferromagnetic powder contained in the magnetic layer 4. Further, thesame surface treatment as could be given to the ferromagnetic metalpowder may be effected.

[0092] The aforementioned nonmagnetic powder is explained. Thenonmagnetic powder includes particles of nonmagnetic iron oxide (redoxide), barium sulfate, zinc sulfide, magnesium carbonate, calciumcarbonate, calcium oxide, zinc oxide, magnesium oxide, magnesiumdioxide, tungsten disulfide, molybdenum disulfide, boron nitride, tindioxide, silicon carbide, cerium oxide, corundum, artificial diamond,garnet, siliceous stone, silicon nitride, molybdenum carbide, boroncarbide, tungsten carbide, titanium carbide, diatomaceous earth,dolomite, and resins. Preferred of them are nonmagnetic iron oxide (redoxide), titanium oxide, and boron nitride. These nonmagnetic powders canbe used either individually or as a combination of two or more thereof.The nonmagnetic particles may have any of a spherical shape, a tabularshape, and an acicular shape or may be amorphous. Spherical, tabular,and amorphous particles preferably have a particle size of S to 200 nm,and acicular particles preferably have a major axis length of 20 to 300nm with an acicular ratio of 3 to 20. Where the nonmagnetic powder isused in combination with the magnetic powder (i.e., where theintermediate layer 3 is a magnetic layer), the nonmagnetic powder ispreferably used in an amount of 30 to 70 parts by weight, particularly40 to 60 parts by weight, per 100 parts by weight of the magneticpowder. Where, on the other hand, the magnetic powder is not used (i.e.,where the intermediate layer 3 is a nonmagnetic layer), the amounts ofthe other components are decided based on 100 parts by weight of thenonmagnetic powder. If necessary, the above-mentioned variousnonmagnetic powders can be subjected to the same surface treatment ascould be done on the magnetic powder.

[0093] The intermediate layer 3, either magnetic or nonmagnetic, cancontain a binder in addition to the above-mentioned components and mayfurther contain abrasive grains, lubricants, carbon black, hardeners,and so forth. While not described specifically, these components can beof those useful in the backcoating layer 5 and magnetic layer 4.Preferred amounts of these components are shown below, given in terms ofparts by weight per 100 parts by weight of the total amount of themagnetic powder and the nonmagnetic powder (where the intermediate layer3 is a magnetic layer) or 100 parts by weight of the nonmagnetic powder(where the intermediate layer 3 is a nonmagnetic layer).

[0094] Binder: 8 to 40 parts by weight, particularly 10 to 25 parts byweight

[0095] Abrasive grains: 1 to 30 parts by weight, particularly 1 to 12parts by weight

[0096] Lubricant: 0.5 to 20 parts by weight, particularly 1 to 7 partsby weight

[0097] Carbon black: 0.5 to 30 parts by weight, particularly 2 to 10parts by weight

[0098] Hardener: 0.5 to 12 parts, particularly 2 to 8 parts by weight

[0099] If desired, the intermediate layer 3 can contain the additives ascould be added to the magnetic layer 4.

[0100] The intermediate layer 3 is formed by coating the substrate 2with an intermediate layer coating composition containing theaforementioned components and a solvent. The solvent may be of thoseused in the backcoating composition and the magnetic coatingcomposition. The amount of the solvent to be used is preferably 100 to700 parts by weight, particularly 300 to 500 parts by weight, per 100parts by weight of the total of the magnetic powder and the nonmagneticpowder (where the intermediate layer 3 is a magnetic layer) or 100 partsby weight of the nonmagnetic powder (where the intermediate layer 3 is anonmagnetic layer).

[0101] The intermediate layer 3 should have some thickness to controlthe capacity of holding lubricants which is influential on thedurability of the magnetic tape 1, but too large a thickness is liableto cause crack initiation when deflected. Accordingly, a preferredthickness is 0.5 to 10 μm, particularly 0.1 to 3 μm.

[0102] Where the intermediate layer 3 is a layer having magnetism, itscoercive force preferably ranges from 80 to 350 kA/m, particularly 150to 300 kA/m, from the standpoint of overwrite characteristics and theoutput balance over a low to high frequency region. Its saturation fluxdensity is preferably 0.02 to 0.1 T, particularly 0.03 to 0.09 T becausetoo high a saturation flux density can result in deterioration of theoverwrite characteristics, which leads to increased noise, and too low asaturation flux density can result in insufficient output.

[0103] Materials constituting the substrate 2 are nonmagnetic materialsincluding polymers, such as polyesters, such as polyethyleneterephthalate, polybutylene terephthalate, polyethylene naphthalate,polycyclohexylene dimethylene terephthalate, and polyethylenebisphenoxycarboxylate; polyolefins, such as polyethylene andpolypropylene; cellulose derivatives, such as cellulose acetate butyrateand cellulose acetate propionate; vinyl resins, such as polyvinylchloride and polyvinylidene chloride; polyamide; polyimide;polycarbonate; polysulfone; polyether ether ketone; and polyurethane.These materials can be used individually or in combination of two ormore thereof. If necessary, the substrate made of these materials can besubjected to uniaxial or biaxial stretching, a corona dischargetreatment, a treatment for improving adhesion, and the like.

[0104] The thickness of the substrate 2 is not particularly limited. Itis preferably 2 to 100 μm, still preferably 2 to 76 μm.

[0105] The outline of a preferred process for producing the magnetictape 1 shown in FIG. 1 is described below.

[0106] A magnetic coating composition for forming the magnetic layer 4and an intermediate layer coating composition for forming theintermediate layer 3 are applied simultaneously to the substrate 2 in awet-on-wet coating system to provide coating layers corresponding to themagnetic layer 4 and the intermediate layer 3 having respectiveprescribed thicknesses. That is, the magnetic layer 4 is preferablyprovided while the intermediate layer 3 is wet.

[0107] The coating layers are then subjected to magnetic fieldorientation, dried, and wound. Thereafter, the coated material iscalendered, and a backcoating layer 5 is formed. Alternatively,formation of the intermediate layer 3 and the magnetic layer 4 may bepreceded by formation of the backcoating layer 5. The coated material isaged at 40 to 80° C. for 6 to 100 hours and then slit to a prescribedwidth. A color change pattern 10 is then formed on the backcoating layer5 in accordance with the aforementioned method.

[0108] The simultaneous coating technique in a wet-on-wet coating systemis described in Japanese Patent Laid-Open No. 73883/93, column 42, line31 to column 43, line 31. This is a technique in which a magneticcoating composition is applied before an intermediate layer coatingcomposition dries. This technique being followed, there is obtainedmagnetic tape which causes few dropouts and can cope with high-densityrecording, and the coating layers of the resulting magnetic tape haveexcellent durability.

[0109] The magnetic field orientation treatment is carried out beforeeach coating composition dries. The treatment can be performed byapplying a magnetic field of about 40 kA/m or higher, preferably about80 to 800 kA/m, in parallel with the side coated with the magneticcoating composition or passing the coated material through a solenoidtype magnet of about 80 to 800 kA/m while the magnetic coatingcomposition is wet. By the magnetic field orientation treatment undersuch conditions, the ferromagnetic powder in the magnetic layer 4 areorientated in the longitudinal direction of the magnetic tape 1. For thepurpose of inhibiting the thus orientated ferromagnetic powder fromchanging its orientation during the subsequent drying step, it is apreferred manipulation to apply warm air of 30 to 50° C. from above themagnetic layer 4 immediately before the magnetic field orientationtreatment, whereby the coated material is dried preliminarily to have acontrolled residual solvent content in each layer.

[0110] The drying of the coating layers is carried out by, for example,supplying gas heated to 30 to 120° C. The degree of drying can becontrolled by adjusting the temperature and the feed rate of the gas.

[0111] The calendering is carried out by, for example, supercalenderingcomprising passing the coated film between two rolls, such as acombination of a metal roll and a cotton roll or a synthetic resin rollor a pair of metal rolls. The calendering conditions are preferably 60to 140° C. in temperature and 100 to 500 kg/cm in linear pressure.

[0112] In the production of the magnetic tape 1, the surface of themagnetic layer 4 can be subjected to a finishing step, such asburnishing or cleaning, according to necessity. It is also possible toapply the magnetic coating composition and the intermediate layercoating composition by a generally known successive coating technique.

[0113] While the magnetic tape of the present invention has beendescribed based on the preferred embodiments thereof it should beunderstood that the present invention is not deemed to be limitedthereto, and various changes and modifications can be made thereinwithout departing from the spirit and scope of the present invention.

[0114] For example, the color change pattern 10 in the foregoingembodiments may be a combination of a color change pattern 10 composedof one continuous line or a plurality of continuous lines having aprescribed width along the longitudinal direction of the magnetic tape 1and a color change pattern 10 composed of discontinuous lines having aprescribed width arranged along the longitudinal direction of themagnetic tape 1.

[0115] The color change pattern 10 may be composed of dots arranged in aline or a curve or a combination thereof.

[0116] Further, the color change pattern 10 may comprise circles,ellipses or any other figures or an arbitrary combination thereof.

[0117] The magnetic tape 1 shown in FIG. 1 can have a primer layerbetween the substrate 2 and the intermediate layer 3 or the backcoatinglayer 5.

[0118] While the magnetic tape according to the above-describedembodiments is of particulate type, the effects of the present inventioncan be produced equally when the present invention is applied tomagnetic tape of metal-deposited type.

[0119] The present invention has been described and will be betterunderstood from the following Examples. However, the Examples are givenfor illustrative purposes only, and the present invention is notconstrued as being limited thereto unless otherwise noted. In Examplesand Comparative Examples, the viscosity of the backcoating compositionswas adjusted by varying the amount of the solvent (methyl ethylketone:toluene:cyclohexanone=3:2:1) so that it may fall within ±30% ofthe viscosity of the backcoating composition of Example 1 (as measuredwith an E type viscometer at 100 rpm) taken as a standard. Unlessotherwise specified, all the parts and percents are given by weight.

EXAMPLE 1

[0120] The following components except the hardener were kneaded in akneader, dispersed in a stirrer, and further finely dispersed in a sandmill. The dispersion was filtered through a 1 μm filter, and finally,the hardener was added thereto to prepare a backcoating composition, amagnetic coating composition, and an intermediate layer coatingcomposition having the respective formulations described below.

[0121] Formulation of Backcoating Composition: FeO_(x) 70 parts (primaryparticle size: 32 nm; BET specific surface area: 52 m²/g; coerciveforce: 10.3 kA/m (129 Oe); saturation magnetization: 85 Am²/kg; divalentFe content: 19.7%; x = 1.363) Phosphoric ester (lubricant) 2 parts(Phosphanol RE610 (trade name), produced by Toho Chemical Industry Co.,Ltd.) Carbon black 1 part (primary particle size: 54 nm; BET specificsurface area: 32 m²/g; DBP oil absorption: 180 cm³/100 g) Indium-dopedtin oxide (ITO) 30 parts (primary particle size: 35 nm) Polyurethaneresin (binder) 17 parts (number average molecular weight: 25000;sulfoxyl group content: 1.2 × 10⁻⁴ mol/g; glass transition point: 45°C.) Stearic acid (lubricant) 1 part Polyisocyante (hardener) 4 parts(Coronate L (trade name) produced by Nippon Polyurethane Industry Co.,Ltd.; solid content: 75%) Methyl ethyl ketone (solvent) 90 parts Toluene(solvent) 60 parts Cyclohexanone (solvent) 30 parts

[0122] Formulation of Magnetic Coating Composition: Acicularferromagnetic metal powder mainly 100 parts comprising iron(Fe:Co:Al:Y:Ba = 70:25:2:2:1 (by weight)) (major axis length: 0.07 μm;acicular ratio: 6; coercive force: 160 kA/m (2010 Oe); saturationmagnetization: 142 Am²/kg; specific surface area: 56 m²/g; X-rayparticle size: 0.014 μm) Alumina (abrasive) 8 parts (primary particlesize: 0.15 μm) Carbon black (antistatic agent) 0.5 part (primaryparticle size: 0.018 μm) Vinyl chloride copolymer (binder) 10 parts(average degree of polymerization: 280; epoxy content: 1.2 wt %;sulfoxyl group content: 8 × 10⁻⁵ equiv./g) Polyurethane resin (binder) 7parts (number average molecular weight: 25000; sulfoxyl group content:1.2 × 10⁻⁴ equiv./g; glass transition point: 45° C.) Stearic acid(lubricant) 1.5 parts 2-Ethylhexyl oleate (lubricant) 2 partsPolyisocyanate (hardener) 5 parts (Coronate L (trade name), produced byNippon Polyurethane Industry Co., Ltd.) Methyl ethyl ketone 120 partsToluene 80 parts Cyclohexanone 40 parts

[0123] Formulation of Intermediate Layer Coating Composition: α-Fe₂O₃100 parts (average particle size (major axis length): 0.12 μm; acicularratio: 10; specific surface area: 48 m²/g) Alumina (abrasive) 3 parts(primary particle size: 0.15 μm) Vinyl chloride copolymer (binder) 12parts (average degree of polymerization: 280; epoxy content: 1.2 wt %;sulfoxyl group content: 8 × 10⁻⁵ equiv./g) Polyurethane resin (binder) 8parts (number average molecular weight: 25000; sulfoxyl group content:1.2 × 10⁻⁴ equiv./g; glass transition point: 45° C.) Stearic acid(lubricant) 1 part 2-Ethylhexyl oleate (lubricant) 4 partsPolyisocyanate (hardener) 4 parts (Coronate L (trade name), produced byNippon Polyurethane Industry Co., Ltd.) Methyl ethyl ketone 90 partsToluene 60 parts Cyclohexanone 30 parts

[0124] The intermediate layer coating composition and the magneticcoating composition were applied simultaneously onto a 6 μm thickpolyethylene terephthalate film substrate by means of a die coater toform the respective coating layers having a dry thickness of 1.5 μm and0.2 μm, respectively. The coated film was passed through a solenoid typemagnet of 400 kA/m while wet and then dried in a drying oven by applyinghot air at 80° C. at a rate of 10 m/min. After the drying, the coatedfilm was calendered to form an intermediate layer and a magnetic layer.Subsequently, the reverse side of the substrate was coated with thebackcoating composition and dried at 90C. to form a backcoating layerhaving a thickness of 0.5 μm. The magnetic tape stock thus obtained wasslit to a width of 12.7 mm to obtain a magnetic tape. The magnetic layerof the resulting magnetic tape had a coercive force of 165 kA/m, asaturation flux density of 0.37 T, and a squareness ratio of 0.86. Thearithmetic mean roughness Ra was 4.2 nm, and the 10 point mean roughnessRz was 38 nm.

[0125] As shown in FIG. 2, the backcoating layer of the resultingmagnetic tape was irradiated with laser beams having a wavelength of1.03 μm, an output of 0.3 W, and a beam diameter of 18 μm to form acolor change pattern of a plurality of lines. The color change patternthus formed was composed of a plurality of continuous straight linesextending in the longitudinal direction of the magnetic tape and equallyspaced in the width direction of the magnetic tape.

EXAMPLE 2

[0126] A magnetic tape was obtained in the same manner as in Example 1,except for using carbon black having a primary particle size of 28 nm, aBET specific surface area of 70 m²/g, and a DBP oil absorption of 50cm³/100 g as the carbon black in the backcoating composition used inExample 1. A color change pattern was formed on the backcoating layer ofthe magnetic tape in the same manner as in Example 1.

EXAMPLE 3

[0127] A magnetic tape was obtained in the same manner as in Example 1,except that the amount of the FeO_(x) of the backcoating compositionused in Example 1 was changed to 100 parts and that ITO was notincorporated. A color change pattern was formed on the backcoating layerof the magnetic tape in the same manner as in Example 1.

EXAMPLE 4

[0128] A magnetic tape was obtained in the same manner as in Example 1,except for replacing ITO of the backcoating composition used in Example1 with Mn—Zn ferrite (primary particle size: 32 nm; BET specific surfacearea: 45 m²/g; Fe₂O₃:MnO:ZnO=70:21:10). A color change pattern wasformed on the backcoating layer of the magnetic tape in the same manneras in Example 1.

EXAMPLE 5

[0129] A magnetic tape was obtained in the same manner as in Example 1,except for replacing the carbon black of the backcoating compositionused in Example 1 with 0.5 part of silicone resin particles(alkyl-modified silicone resin particles; primary particle size: 300nm). A color change pattern was formed on the backcoating layer of themagnetic tape in the same manner as in Example 1.

COMPARATIVE EXAMPLE 1

[0130] A magnetic tape was obtained in the same manner as in Example 1,except for replacing the FeO_(x) of the backcoating composition used inExample 1 with α-Fe₂O₃ (average particle size (major axis length): 0.12μm; aspect ratio: 10; specific surface area: 48 m²/g). A color changepattern was formed on the backcoating layer of the magnetic tape in thesame manner as in Example 1.

COMPARATIVE EXAMPLE 2

[0131] A magnetic tape was obtained in the same manner as in Example 1,except for replacing 70 parts of FeO_(x) of the backcoating compositionused in Example 1 with 10 parts of carbon black (primary particle size:28 nm; BET specific surface area: 70 m²/g; DBP oil absorption: 50cm³/100 g). A color change pattern was formed on the backcoating layerof the magnetic tape in the same manner as in Example 1.

COMPARATIVE EXAMPLE 3

[0132] A magnetic tape was obtained in the same manner as in Example 1,except for changing the amount of the polyurethane resin of thebackcoating composition used in Example 1 to 50 parts. A color changepattern was formed on the backcoating layer of the magnetic tape in thesame manner as in Example 1.

COMPARATIVE EXAMPLE 4

[0133] A magnetic tape was obtained in the same manner as in Example 1,except for changing the amount of the polyurethane resin of thebackcoating composition used in Example 1 to 10 parts. A color changepattern was formed on the backcoating layer of the magnetic tape in thesame manner as in Example 1.

[0134] In order to evaluate the performance of the magnetic tapesobtained in Examples and Comparative Examples, the reproduction outputof the magnetic tape, the arithmetic mean roughness Ra, 10 point meanroughness Rz, coefficient of dynamic friction, surface resistivity, voiddiameter, and void volume of the backcoating layer, and the lighttransmission and color change of the backcoating layer were measured.Further, the magnetic tape was subjected to a servo signal writing test.The results are shown in Table 1. The P/B ratio of the backcoating layeris also shown in Table 1. Of these measurements, the arithmetic meanroughness Ra, 10 point mean roughness Rz, void diameter and void volumeof the backcoating layer were measured in accordance with theaforementioned methods. The other measurements were made according tothe following methods.

[0135] Reproduction Output:

[0136] A head tester method was followed. Signals having a recordingwavelength of 0.6 μm were recorded, and the reproduction output wasmeasured. The results obtained were expressed relatively takingComparative Example 1 as a standard (0 dB).

[0137] Coefficient of Dynamic Friction (μ):

[0138] The magnetic tape was run on a tape tester TBT-300D manufacturedby Yokohama System Kenkyusho K.K. at a speed of 3.36 cm/sec with itsmagnetic layer in contact with a cylinder having a diameter of 5 mm at180°. The tensions on the reel-off side and the reel-up side weremeasured to obtain a frictional coefficient (μ) from equation (iii):

μ=(1/π)1n(reel-off tension)/(reel-up tension)  (iii)

[0139] Surface Resistivity:

[0140] A pair of electrodes plated with 24-K gold and finished to have asurface roughness of N4 (see ISO 1302) and having a radius of 10 mm wereput in parallel horizontally on the magnetic layer with acenter-to-center distance d=12.7 mm. A direct voltage of 100 V±10 V wasapplied to the electrodes while applying a force of 0.25 N to both endsof the magnetic tape, and the current between the electrodes wasmeasured, from which the surface resistivity was obtained.

[0141] Light Transmission:

[0142] The magnetic tape was irradiated with monochromatic light havinga wavelength of 900 nm, and the percent light transmission in terms ofthe ratio of transmitted light to incident light was obtained. Thevalues shown in Table 1 are transmissions measured before theirradiation with a laser.

[0143] Color Change in Color Change Pattern Area:

[0144] The part irradiated with laser beams was observed with the nakedeye and under an optical microscope.

[0145] Servo Tracking Test:

[0146] Signals were recorded on the magnetic layer of the magnetic tapefor evaluation while carrying out servo tracking in accordance with apush-pull method. The servo signals were detected by converting thedifference in light transmission at 1030 nm between a discolored partand a non-discolored part of the backcoating layer into electricalsignals. TABLE 1 Backcoating Layer Color Change Pattern Dynamic SurfaceVoid Void Light Servo Reproduction Ra Rz Friction Resistivity DiameterVolume P/B Transmission*¹ Color Tracking Output (dB) (nm) (nm)Coefficient (Ω/□) (nm) (%) Ratio (%) Change Test Ex. 1 +0.6 11 85 0.214.2 × 10⁶ 5.6 28 5.05 24 observed OK Ex. 2 +0.3 9.4 58 0.42 5.1 × 10⁶4.7 27 5.05 26 observed OK Ex. 3 +0.4 8.6 81 0.28 7.3 × 10⁹ 5.2 32 5.0521 observed OK Ex. 4 +0.2 9.1 71 0.26 4.7 × 10⁹ 6.6 26 5.05 18 observedOK Ex. 5 +0.3 10 87 0.23 4.6 × 10⁶ 6.0 28 5.03 27 observed OK Comp. 0(standard) 8.8 76 0.28 ≧10¹² 5.1 21 5.05 37 not NG Ex. 1 observed Comp.−0.2 17 122  0.20 6.3 × 10⁵ 7.3 39 2.05 16 not NG Ex. 2 observed Comp.+0.1 13 72 0.46 3.8 × 10⁹ 4.6 9.5 1.91 32 not NG Ex. 3 observed Comp.−0.3 34 215  0.27 5.6 × 10⁸ 8.3 44 7.77 18 observed NG*² Ex. 4

[0147] As is apparent from the results shown in Table 1, the magnetictapes of Examples (samples according to the present invention) arecapable of reliable servo tracking without suffering from impairment ofthe functions essential to the backcoating layer. In particular, themagnetic tapes of Examples achieved reliable servo tracking even whenthe tapes were recorded on data tracks of 600 tpmm as demonstrated inTable 1.

[0148] Industrial Applicability:

[0149] As described in detail the present invention provides magnetictape which is capable of servo tracking without reducing the data area.

[0150] The present invention provides magnetic tape which is capable ofservo tracking without suffering from impairment of the functionessential to the backcoating layer.

[0151] The present invention provides magnetic tape having an improvedtrack density.

[0152] The present invention provides magnetic tape having a highrecording capacity.

[0153] It is apparent from the above teachings that variousmodifications can be made in the present invention. Accordingly, itshould be understood that the invention can be practiced otherwise thanas specifically described within the scope of the appended claims.

1. Magnetic tape comprising a substrate, a magnetic layer provided onone side of said substrate and a backcoating layer provided on the otherside of said substrate, wherein: said backcoating layer comprises abinder and fine particles having been dispersed in said binder and beingcapable of irreversibly changing in color on oxidation reaction, and hasa sufficient number of microvoids of sufficient size to supplysufficient oxygen to cause said oxidation reaction.
 2. Magnetic tape asclaimed in claim 1, wherein said backcoating layer is to be irradiatedwith a light beam to cause oxidation of said fine particles whereby saidfine particles undergo color change to form a color change pattern ofprescribed form on the backcoating layer so that servo tracking of datatracks on said magnetic layer can be carried out based on the opticalinformation provided from said color change pattern.
 3. Magnetic tape asclaimed in claim 1, wherein the void volume of said microvoids in saidbackcoating layer is 5 to 40% by volume.
 4. Magnetic tape as claimed inclaim 1, wherein said fine particles comprise a metal oxide having aprimary particle size of 1 to 200 nm.
 5. Magnetic tape as claimed inclaim 4, wherein said metal oxide comprises FeO_(x) (1.34<x<1.5), TiO,SnO, MnO or Cr₂O₃.
 6. Magnetic tape as claimed in claim 1, wherein saidbackcoating layer has an arithmetic mean roughness Ra of 7 to 50 nm anda 10 point mean roughness Rz of 40 to 250 nm.
 7. Magnetic tape asclaimed in claim 1, wherein said backcoating layer contains 0.1 to 5parts by weight of carbon black per 100 parts by weight of said binder,said carbon black having a primary particle size of 15 to 80 nm, a BETspecific surface area of 10 to 80 m²/g, and a DBP oil absorption of 100to 300 cm³/100 g.
 8. Magnetic tape as claimed in claim 1, wherein saidbackcoating layer contains 0.05 to 10 parts by weight of silicone resinparticles having a primary particle size of 10 to 500 nm per 100 partsby weight of said binder.
 9. Magnetic tape as claimed in claim 1,wherein said backcoating layer contains 5 to 100 parts by weight ofelectrically conductive inorganic particles having a primary particlesize of 1 to 100 nm per 100 parts by weight of said fine particles. 10.Magnetic tape as claimed in claim 9, wherein said electricallyconductive inorganic particles comprise tin oxide, antimony-doped tinoxide, indium-doped tin oxide or indium oxide.
 11. Magnetic tape asclaimed in claim 1, wherein said color change pattern comprises a singleor a plurality of continuous lines having a prescribed width along thelongitudinal direction of the tape.
 12. Magnetic tape as claimed inclaim 1, wherein said color change pattern comprises discontinuouspieces of lines having a prescribed width along the longitudinaldirection of the tape.
 13. Magnetic tape as claimed in claim 1, whereinservo tracking is carried out by detecting reflected light of the lightincident on said color change pattern.
 14. Magnetic tape as claimed inclaim 1, wherein servo tracking is carried out by detecting transmittedlight of the light incident on said color change pattern.
 15. Magnetictape as claimed in claim 1, wherein at least one magnetic or nonmagneticintermediate layer is provided between said substrate and said magneticlayer, and said magnetic layer comprises acicular or spindle-shapedferromagnetic metal powder having a major axis length of 0.03 to 0.2 μmor tabular ferromagnetic hexagonal ferrite powder having a tabulardiameter of 0.1 μm or smaller.
 16. Magnetic tape comprising a substrate,a magnetic layer provided on one side of said substrate and abackcoating layer provided on the other side of said substrate, wherein:said backcoating layer comprises a binder and fine particles havingdispersed in said binder and being capable of irreversibly changing incolor on oxidation reaction, and has a sufficient number of microvoidsof sufficient size to supply sufficient oxygen to cause said oxidationreaction, and said fine particles have changed in color to form a colorchange pattern of prescribed form on said backcoating layer.